1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus using a periodic wave as a developing bias.
2. Description of the Related Art
With an electrophotographic image forming apparatus, a developing bias (blank pulse) having a wave formed by a pulse section and a pausing section (blank section) is applied to a developing device. Although a voltage in which an AC voltage is superimposed on a DC voltage is applied in the pulse section, in the blank section, the AC voltage is not superimposed and only the DC voltage is applied. It has been confirmed that the blank pulse is effective in improving image quality, and thus has been used in many instances (U.S. Pat. No. 6,459,862).
The blank pulse is advantageous in terms of the following three aspects.
1) In the pulse section, toner present between a photosensitive body and a developing device repeatedly adsorbs (a process of returning from the photosensitive body to the developing device) and discharges (a process of adhering to the photosensitive body from the developing device). This repetition of adsorption and discharge prevents toner whose developing performance is unstable from adhering to the photosensitive body, which leads to an improvement in image quality.
2) A potential difference between the surface of the developing device and that of the photosensitive body is constant at the peak of the amplitude in the pulse section. Therefore, the adsorption and the discharge are repeatedly executed in a stable manner, which leads to an improvement in image quality.
3) By including a blank section in the developing bias, a certain time period for discharging toner is secured. Accordingly, it is possible to readily stabilize developing performance, which is advantageous for improving image quality.
Incidentally, it becomes possible to readily generate a square wave having a good waveform by disposing a resistor on the secondary side of a boosting transformer for generating a developing bias. However, such a resistor causes loss of electric power (generation of heat). Particularly when images are formed in succession, the temperature rises greatly due to the resistor. In view of this, the following methods are possible for mitigating such rising of the temperature.
i) A cooling mechanism such as a fan for cooling a developing bias output circuit is disposed.
ii) Continuous image formation is interrupted, thereby suppressing temperature rising in a developing bias supply circuit.
iii) The duration of the blank section at the time of temperature rising is set to be longer than the duration of the blank section during normal image formation, thereby suppressing temperature rising in the developing bias supply circuit.
However, employing a cooling fan not only invites an increase in cost, but also makes it difficult to reduce the size of the image forming apparatus. Also, when continuous image formation is interrupted, the downtime (waiting period in which image formation cannot be performed) of the image forming apparatus increases. In other words, image productivity and user convenience will be reduced. The method in which the duration of the blank section is increased is advantageous in that it has little effect on the potential amount, toner carriage amount, or the like, of the photosensitive body, on which the quality of output images depends. However, as the duration of the blank section becomes longer, the development period becomes longer. As a result, “fogging” (the discharge of toner particles having unstable developing performance) often occurs, which reduces the service life of an image forming unit.